KABUL, Afghanistan (CNN) -- A NATO airstrike on a pair of hijacked fuel trucks early Friday in northern Afghanistan killed at least 90 people -- a mix of Taliban militants and civilians, NATO and provincial officials said.

A victim of an ISAF airstrike on a hijacked oil tanker is carried into the Kunduz hospital on Friday.

Capt. Elizabeth Mathias of NATO's International Security Assistance Force, also known as ISAF, said the military believed there were no civilians near the trucks at the time of the attack.

NATO learned afterward that was not the case. "Based on what we know, there were civilians there," Mathias said.

Local Afghan officials were quoted as saying in some news reports that nearly half the people killed in the airstrike were civilians who rushed the fuel trucks. Mathias declined to confirm those numbers.

Investigators are trying to determine who was at the site and whether a mistake was made, Mathias said. She said a local NATO team is already in Kunduz province conducting an investigation with Afghan forces and another team from the central office is on its way. Watch more about the airstrikes »

"Because of the prevalence of reports of civilian casualties, we don't want to be seen as ignoring the situation," Mathias said. "We don't want to wait. If something happened, we want to apologize."

Gen. Stanley McChrystal, the top U.S. commander in Afghanistan, is following the investigation, his public affairs officer, Lt. Col. Tadd Sholtis, said in an e-mail.

"He takes the possible loss of any innocent life seriously, and while he will not rush to judgment on the facts, he is following the investigation very closely," Sholtis said.

Brigadier General Eric Tremblay released a statement that said, "ISAF will do whatever is necessary to help the community, including medical assistance and evacuation as requested. ISAF regrets any unnecessary loss of human life, and is deeply concerned for the suffering that this action may have caused to our Afghan friends." See images of the strike aftermath »

Afghan President Hamid Karzai said he was "deeply saddened" by the deaths and reiterated that no civilians should be killed or injured in anti-terrorist military operations.

Sholtis said McChrystal "has been in touch with President Karzai and leadership of the major ministries on the incident."

Karzai's office issued a statement saying he had ordered a delegation to the area to investigate the incident and report back to him as soon as possible. The delegation includes representatives of the Ministry of Interior Affairs, National Directorate of Security, and the Provincial Administrative Department, according to the statement.

A spokesman for the provincial governor, Mahboobullah Sayeedi, said more than 90 people were killed.

The fuel trucks were hijacked late Thursday in Kunduz province and were spotted several hours later on the banks of the Kunduz River, ISAF said.

The hijacked vehicles became stuck in the Chardara area of the Ali Abaad district while trying to cross the river, according to Sayeedi. Militants had commandeered the trucks, which were carrying fuel for NATO forces, he said. People tried to empty fuel from the tankers when they couldn't go any farther.

With the trucks stuck on the riverbank, the German commander of the NATO forces called in the airstrike around 2:30 a.m., the German military said.

No German soldiers or planes were involved in the airstrike, but a German patrol made it to the site about 10 hours after the attack and came under small-arms fire, the German military said. The patrol continued its investigation Friday afternoon.